Implantable medical devices often include an electrode to which a conductive wire is attached. Conventionally, medical lead wires are simply welded to an electrode. However, there are several disadvantages to this technique: (1) lead wire and electrode materials are often dissimilar and are at risk of sustaining galvanic corrosion when used together in an in vivo environment in which they are exposed to bodily fluids; (2) if lead wire and electrode materials are dissimilar, welds between such dissimilar materials are typically of lower quality than welds between similar materials; (3) lead wire diameters for implanted medical devices are generally small, so that properly welding these small-diameter lead wires to electrodes can present manufacturing challenges; and (4) laser spot welding of such relatively small diameter lead wires can lead to poor quality welds.
With respect to the dissimilar materials noted above, because lead wires and electrodes serve different functions, the properties desired for the lead wire material may be different than the properties desired for the electrode material. Accordingly, a desirable material for a lead wire might not be desirable for an electrode. For example, lead wires for implanted medical devices must exhibit a high tensile strength, to withstand repeated bending cycles without breakage, and a nickel cobalt alloy (MP35N) is often selected as a material for lead wires. Because the lead wires are encapsulated in an insulating polymer cover, the biocompatibility of the lead wire material is generally less important than the material's tensile strength. The electrodes, however, are often placed in contact with bodily fluids or tissue, and thus, the biocompatibility of the material from which the electrodes are formed is quite important. Platinum-iridium alloys are biocompatible, inert, and very conductive; thus, electrodes in medical devices are often made from such alloys.
Where two different metals are connected together and exposed to a common electrolyte (in the case of an implanted medical device, bodily fluid represents the common electrolyte), there exists a risk of galvanic corrosion, which is highly undesirable in the context of the long term reliability of an implantable medical device.
With respect to the manufacturing challenges noted above, in the context of implanted medical devices, lead wire diameters can be as small as about 0.003 inches (˜75 microns), which is approximately the diameter of a single human hair, or even as little as 0.001 inches. Aligning such small diameter lead wires for welding on a relatively larger electrode can be challenging, especially with regards to multifilar wires having eight or more individual wire strands.
With respect to problems associated with laser spot welding of relatively small diameter lead wires, conventional laser spot welding does not utilize an additional filler material, so the material that is liquefied to make the weld is produced by melting the components being welded together. In the case of a relatively small diameter lead wire, the lead wire will melt well before the electrode (because the electrode is more massive than the lead wire), and the lead wire can either melt entirely (leading to immediate failure of the weld), or the diameter of the lead wire can be greatly reduced when melted, generating a weak spot that is prone to failure. A further problem is that where the lead wire material has been initially heat treated, the heat from the welding operation can adversely affect the initial heat treatment, reducing the strength of the lead wire.
Based on the disadvantages of the conventional approach used to join lead wires to electrodes discussed above, it would clearly be desirable to provide alternative techniques for coupling lead wires to electrodes in implantable medical devices.